1. Field of the Invention
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2006-225932, filed on Aug. 22, 2006, the disclosure of which is incorporated herein in its entirety by reference.
The present invention relates to a radio communications system and, more particularly, to a radio communications system employing a scheme of multiplexing reference signals (also referred to as pilot signals) with transmission signals, as well as a technique for multiplexing reference signals, and radio communication equipment using the technique.
2. Description of the Related Art
In general, since transmission signals are under the influence of radio channel fading, radio communications systems employ a scheme of multiplexing a reference signal with a transmission signal. That is, a reference signal received is used to perform channel estimation for correct modulation/detection (hereinafter, “modulation/detection” will mean modulation, detection, or modulation and detection), and to perform channel quality (CQI: Channel Quality Indicator) measurement for link adaptation or scheduling.
Particularly in a mobile communications system in which a base station carries out channel-dependent scheduling for a plurality of mobile stations, since a resource is generally allocated to a mobile station exhibiting the best CQI, CQI measurement is performed in the entire frequency band where data may be transmitted, with respect to those mobile stations waiting for resource allocation. For CQI measurement, utilized is a reference signal multiplexed on an uplink that the base station receives from each mobile station. In the case where a reference signal for demodulation of an uplink data signal or uplink control signal is multiplexed, this reference signal can also be utilized for CQI measurement.
To perform channel estimation and the like by using a reference signal, the receiving side also needs to know in advance a reference signal sequence to be transmitted. For such a sequence, CAZAC (Constant Amplitude Zero Auto-Correlation) sequence has been attracting attention in recent years. The CAZAC sequence has the characteristics that the peak-to-average power ratio (PAPR) can be kept low because the amplitude is constant in time domain, and that excellent channel estimation in frequency domain is possible because the amplitude is constant also in frequency domain (for example, see Fazel, K., and Keiser, S., “Multi-Carrier and Spread Spectrum Systems,” John Willey and Sons, 2003). Therefore, the CAZAC sequence is used as uplink reference-signal sequences also in the 3GPP Long Term Evolution (see 3GPP TR 25.814 v2.0.0, June, 2006).
Such a reference signal is periodically multiplexed in every frame so that variations due to channel fading can be accurately estimated. In general, for a single channel, a plurality of reference signals transmitted at discrete timings is used to perform channel estimation and CQI measurement.
FIG. 1A is a format diagram showing an example of a frame structure described in 3GPP R1-051033, Motorola, “Further Topics on Uplink DFT-SOFDM for E-UTRA,” Oct. 10-14, 2005. In this example, one frame (sub-frame) has a frame length of 0.5 msec and includes six long blocks LB#1 to LB#6 for transmitting control and data signals and two short blocks SB#1 and SB#2 for transmitting reference signals, with a cyclic prefix (CP) added to each block. That is, the reference signals are time-multiplexed with control and data signals in a frame. The number of short blocks SB to be allocated for reference signals is dependent on the length of a frame. As for the timings of the short blocks SB#1 and SB#2 within a frame, it is sufficient to determine the timings so that the reference signals will function effectively, and the timings shown in the frame structure of FIG. 1A are not limitative.
Moreover, regarding the reference signals, which are allocated the short blocks SB#1 and SB#2, a plurality of orthogonal reference signals can be frequency-multiplexed within a certain frequency band, allowing transmission in a single short block, and these orthogonal reference signals can be allocated to different user equipments respectively. However, the reference-signal bandwidth required by each user equipment is not always the same as that required by another user equipment, and suitable transmission bandwidths differ depending on what purpose a reference signal is used for (such as for modulation/detection of a data signal, for modulation/detection of a L1/L2 control signal, or for CQI measurement).
For example, when a data signal or L1/L2 (physical layer/data link layer) control signal with a transmission bandwidth of 5 MHz is transmitted in a frequency bandwidth of 10 MHz, it is desirable to use a reference signal with the same transmission bandwidth of 5 MHz in order to achieve highly reliable demodulation/detection. However, in the case of a reference signal for CQI measurement, the restriction as to the transmission bandwidth is relaxed because the reference signal is not used for demodulation/detection.
To multiplex as many reference signals as possible while ensuring the orthogonality between the reference signals with different transmission bandwidths as described above, several multiplexing methods have been proposed.
1) Distributed Frequency Division Multiplexing
FIG. 1B is a diagram of a reference-signal structure showing an example of distributed frequency division multiplexing (distributed FDM) of reference signals. Here, it is assumed that a frequency bandwidth of 10 MHz includes four 2.5-MHz frequency blocks, in each of which six subcarriers can be frequency-multiplexed. Moreover, it is assumed that two of the six subcarriers in each frequency block are assigned to each of three transmission bandwidths Δf(a), Δf(b), and Δf(c).
In this example, a set of distributed reference signals corresponding to the transmission bandwidth Δf(a) of 10 MHz is allocated to a set of user equipments (UEs) 1a and 2a, in each 2.5-MHz frequency block. Taking the case of the UE 1a as an example, the subcarriers allocated to the UE 1a in the four respective frequency blocks, occupying a four-toothed comb-shaped spectrum, provides one frequency resource. Similarly, two sets of distributed reference signals corresponding to the transmission bandwidth Δf(b) of 5 MHz are respectively allocated to two sets of UEs: UEs 1b and 2b, and UEs 3b and 4b. Further, four sets of distributed reference signals corresponding to the transmission bandwidth Δf(c) of 2.5 MHz are respectively allocated to four sets of UEs: UEs 1c and 2c, UEs 3c and 4c, UEs 5c and 6c, and UEs 7c and 8c. That is, in distributed FDM, the orthogonality between reference signals with different transmission bandwidths can be ensured because even if reference signals have different transmission bandwidths, the reference signals are distributed across the frequency axis.
However, distributed FDM has a demerit that the number of CAZAC sequences that can be secured decreases as the number of reference signals that are multiplexed in a certain frequency band increases. This is because the maximum number of CAZAC sequences that can be secured is obtained by subtracting one (1) from the sequence length (sequence length−1), and the sequence length of each reference signal decreases as the number of reference signals multiplexed in a certain frequency band increases.
For example, in the case where the total of six distributed reference signals with three types of transmission bandwidths Δf of 10 MHz, 5 MHz, and 2.5 MHz (two signals to each type) are multiplexed in each 2.5-MHz bandwidth (frequency block) as shown in FIG. 1B, a frequency component to be allocated to one distributed reference signal is one sixths of a frequency component allocated in the case of a reference signal occupying a continuous 2.5-MHz frequency block (in the case of localized reference signals). Since the sequence length of a reference signal depends on the number of subcarriers, the sequence length of a reference signal is reduced to ⅙ when a frequency component allocated is ⅙. In proportion to this, the number of CAZAC sequences that can be secured is also reduced. Such a reduction in the number of sequences means that the probability of the same sequence being selected by adjacent cells increases when this scheme in question is applied to a mobile communications system.
2) Hybrid Scheme (CDM+Distributed FDM)
To overcome the above-described restrictions as to the number of CAZAC sequences in distributed FDM, a hybrid scheme of code division multiplexing (CDM) and distributed FDM has been proposed (see 3GPP R1-060319, NTT DoCoMo et al., “Orthogonal Pilot Channel Structure for E-UTRA Uplink,” February, 2006). According to this scheme, CDM is used to multiplex reference signals with the same transmission bandwidth, and distributed FDM is used only to multiplex those with different transmission bandwidths. With this scheme, as a whole, the sequence length of each reference signal can be made longer than in the case of using distributed FDM only. Accordingly, the restrictions as to the number of CAZAC sequences can be diminished.
FIG. 1C is a diagram of a reference-signal structure showing an example of the hybrid scheme of CDM and distributed FDM. According to the hybrid scheme, even if the total of six reference signals with three types of transmission bandwidths Δf of 10 MHz, 5 MHz, and 2.5 MHz (two signals to each type) are multiplexed in each 2.5-MHz frequency block as in FIG. 1B, since distributed reference signals with the same transmission bandwidth (here, corresponding to “1a and 2a,” “1b and 2b,” etc.) are code-multiplexed, the number of frequency components that can be allocated to one distributed reference signal is, at the maximum, twice the number in the case of using distributed FDM only as in FIG. 1B. Accordingly, the sequence length becomes twice, and hence the number of CAZAC sequences that can be secured is also proportionately increased.
However, according to the above-described hybrid scheme, since the sequence length is increased by code-multiplexing reference signals with the same transmission bandwidth, this merit cannot be exploited when reference signals are of many types with different transmission bandwidths. That is, when there are a large number of different types of distributed reference signals with different transmission bandwidths, the sequence length of each reference signal is short, and the restrictions as to the number of sequences that can be secured cannot be satisfactorily diminished.